A medical instrument introduction device, includes an outer tube portion indwelled within a body wall; and an inner needle portion inserted through the outer tube portion, wherein the outer tube portion includes a base portion; a tubular conduit portion whose proximal end portion is connected to the base portion; and an anchor portion connected to a distal end portion of the conduit portion, wherein the conduit portion has a bellows folded shape in a natural state, a dimension of the conduit portion in an axial direction being capable of being increased as the folded shape is stretched by the inserted inner needle portion, and a force needed for stretching the folded shape being gradually decreased in a direction from the proximal end portion to a distal end portion of the conduit portion, and the anchor portion and the inner needle potion are couplable with each other.